The present invention has essentially for a subject matter a method of making an externally and internally threaded insert, as well as a tool for carrying out the said method.
It is also directed to an insert obtained by means of such a method and/or such a tool.
It is known that, in order to obtain screwed assemblies, a certain number of internally threaded holes must be machined in a solid workpiece, whereafter any desired element can be attached thereto by screwing. In such assemblies, however, the resistance of the internal thread after the screwing may prove to be too low for them to withstand the tensile force exerted by the screws. Consequently, the co-operating threads of the tapped hole and of the screw may be damaged. The same applies in case of frequent disassemblings of the assembled parts, since, as is readily understood, the threads become worn and therefore deteriorate in the course of time. Moreover, it is extremely difficult, if not impossible, especially as regards the parts used in the aircraft industry, to provide the internally threaded hole with means for self-locking of the screw during the screwing.
Consequently, to overcome these disadvantages, it has already been proposed to use in the screwed assemblies intermediate inserts in the form of small, externally and internally threaded cylindrical tubes.
More precisely, the insert is screwed into the internally threaded hole provided in the solid body, and then set in the said hole by its upper portion, whereafter a screw can be screwed into the insert to fasten any desired element to the said body.
Since the insert is made from a hard material and its external thread is anchored in the internally threaded hole, it is understood that the abovementioned disadvantages resulting from the use of a simple internally threaded hole are removed.
However, problems arise when it is desired to provide such inserts with means for the self-locking of the screw. To this end, the insert may for example be deformed or ovalized prior to its insertion into the internally threaded hole, but such deformation disappears under the action of the introduction and screwing of the insert into the internally threaded hole which is perfectly circular, so that the self-locking effect sought is reduced to nothing. Since it is absolutely necessary, especially in the aircraft industry, that the insert should possess self-locking features, the insert which does not offer the required self-locking features must therefore be extracted from the internally threaded hole by cutting-off its set portion by means of, for example, a small milling cutter, until the said hole is provided with an insert really offering sufficient and efficient self-locking features when screwing the screw.
It is therefore understood that the successive insert interchanging or replacing steps to be carried out until an insert is obtained which, once positioned, offers the required self-locking features, are extremely difficult, time-cnsuming and expensive, to say nothing of the risk of damaging the threads of the threaded hole when cutting off the set portion of the insert and extracting it.